Support more relationship types

[simoncozens]

Currently the SPDX generator assumes that the source of a relationship is always a package, and the target is either a package or a file:

lib4sbom/lib4sbom/generator.py

Lines 174 to 190 in 5a06007

	source_ident = self.bom.package_ident(
	self._get_element(
	relationship["source"], relationship["source_id"]
	)
	)
	if relationship.get("target_type") == "file":
	target_ident = self.bom.file_ident(
	self._get_element(
	relationship["target"], relationship["target_id"]
	)
	)
	else:
	target_ident = self.bom.package_ident(
	self._get_element(
	relationship["target"], relationship["target_id"]
	)
	)

SPDX relationships may occur between any elements:

This field provides information about the relationship between two SPDX elements. For example, you can represent a relationship between two different Files, between a Package and a File, between two Packages, or between one SPDXDocument and another SPDXDocument.